v0.51.3

Synaptic Mesh v0.51 receiver runtime invocation shim rehearsal.

This release adds a local, reversible deterministic receiver runtime invocation shim over pinned v0.50.5 test-harness consumption evidence. It invokes the local shim and prepares no-effect context handoff outputs for human review only.

Boundary preserved: no production runtime integration, SDK/framework/MCP/A2A integration, daemon/watch, tool execution, network/resource fetch, memory/config writes, external effects, authorization/enforcement/allow/block decisions, or agent-consumed authority. The compatibility sentinel remains top-level policyDecision: null only.

Verification included local gates, two independent local reviews (A and B), PR #189 CI, main release check, and exact-tag verification for v0.51.5.